1. Field of the Invention
The present invention relates to an air flow meter for detecting an engine intake air flow rate, used for electronically controlling the engine (hereinafter referred to as "air flow sensor"). More particularly, the invention relates to an air flow sensor which is arranged downstream of an air cleaner and in which the characteristics peculiar to such an air flow sensor are not influenced even when the flow velocity distribution is not uniform over the cross-sectional area of the passage.
2. Description of the Related Art
Conventional air flow sensors are known as disclosed in, for example, Japanese Patent Laid-Open Nos. 63-243721, 58-111720, 62-187219, 58-61411 and 57-103061.
FIG. 12 is a sectional view of the device for detecting the engine intake air flow rate and shows the construction of the in take system disclosed in Japanese Patent Laid-Open No. 63-243721. Referring to FIG. 12, a swirl forming duct 12 is arranged upstream of a hot wire-type air flow sensor 11, and an air cleaner 13 removes dust particles from the intake air before it flows through the swirl forming duct 12.
FIG. 13 is a sectional view of the construction of the intake system disclosed in Japanese Patent Laid-Open No. 58-111720. Referring to FIG. 13, a guide pipe 22 is disposed in an air cleaner 23 arranged upstream of an air flow sensor 21, thereby relaxing turbulence in the intake air caused by a wake produced due to the shape of the portion adjacent to a bladder of the air cleaner 23.
FIG. 14 is a sectional view of the construction of the intake system disclosed in Japanese Patent Laid-Open No. 62-187219. Referring to FIG. 14, a honeycomb flow setting member 32 is fixed to the upstream end of an air flow sensor 31, thereby inhibiting disturbance in an output signal transmitted from the air flow sensor 31.
FIG. 15 is a sectional view of the construction of the intake system disclosed in Japanese Patent Laid-Open No. 58-61411. Referring to FIG. 15, an air cleaner 43 is disposed upstream of a flow setting tube 42 to which an air flow sensor 41 is attached. A flow setting grid 44 is further positioned in the air cleaner 43 and is attached to the upstream end of the flow setting tube 42. It thus sets the air flow from the air cleaner 43, and the flow setting tube 42 also decreases turbulence in the air flow.
FIG. 16 is a sectional view of the construction of the intake system disclosed in Japanese Patent Laid-Open No. 57-103061. Referring to FIG. 16, a bent tube 52 intervenes between an air flow sensor 51 and an air cleaner 53, and a flow setting grid 54 is also attached to the downstream end of the bent tube 52. The bent tube 52 thus alters the direction of the flow from the air cleaner 53, and the flow setting grid 54 calms air turbulence produced in the bent tube 52.
That is, all the conventional air flow sensors described above are arranged downstream of the air cleaners. In order to inhibit air turbulence, the flow setting means sets air flowing from the air cleaner, or the duct through which air flows is specifically constructed.
However, such conventional air flow sensors present the following problems. They cannot inhibit or relax the turbulence in the intake air flow caused by the shape of the air cleaner without providing a structural member for the intake system. The output values detected by the air flow sensor are thus not always reliable and a correct detection of the air flow cannot be performed if the air flow sensor is soiled.
Further, since it is necessary to provide conventional air flow sensors with flow setting means for the intake system and with structural members, such as, a duct through which the set-air flows, the design of the intake system has to be limited by such structural members. More specifically, a space is required to fix structural members, such as a flow setting tube and a guide pipe, within the air cleaner, and consequently, the shape of the air cleaner is restricted, or the structural member such as a duct must be disposed between the air cleaner outlet and the air flow sensor.